Conventional ignition systems employ mechanical breaker points or electronic circuitry driven by rotation of some part of the distributor, typically a distributor shaft, to properly time high voltage pulses which are generated by an ignition coil for firing the spark plugs. The high voltage output of the coil is usually fed through a central electrical contact in the distributor cap to a conductor mounted on a dielectric rotor on the distributor shaft. As the shaft rotates, operating the breaker points or the electronic circuitry connected to the ignition coil primary (low voltage) winding in time with piston travel, the electrical contact at the outer end of the rotor successively engages each of a set of electrical contacts mounted circumferentially on the distributor cap and connected through high voltage leads to each of the spark plugs. The cap electrical contacts and the rotor electrical contact thus constitute a rotating mechanical switching mechanism whose purpose it is to distribute the high voltage signal to each of the spark plugs in the proper time relationship to engine operation. These systems suffer from various shortcomings. The rotor and cap electrical contacts wear so that periodic replacement is required. Those same contacts are particularly vulnerable to water, moisture and other contaminants (such as carbon or other foreign substances), which can prevent starting or lead to poor engine performance, such as misfiring, rough idle and poor fuel economy. Attempts have been made to improve these sytems. In one design a signal rotary element produces a single switching signal which, through complicated electronic circuitry, simultaneously interrupts the ignition coil primary current and controls the high voltage separately applied to each spark plug through a gas tube which apparently conducts in response to a reduction of its breakdown voltage induced by a magnetic field (U.S. Pat. No. 4,019,486). In addition to being complex, this approach is energetically very inefficient, since it requires the generation of rather intense magnetic fields. As a consequence, certain of its components must apparently be rather large. In another design the high voltage is switched independently using a frequency matching scheme, but this approach is also rather complex and cumbersome. In addition, neither of these systems is readily adaptable to existing ignition systems. The magnetically switched system, for example, includes no provision for incorporating the centrifugal and vacuum spark advance mechanisms necessary for efficient engine operation.